While the present invention relates to diode structures in general, for purposes of the following discussion a zener diode structure is addressed. It will be understood, however, that all diode structures are considered within the spirit and scope of the invention.
There are several possible structural arrangements of the P and N regions forming the rectifying junction of a diode or, in this case, the zener breakdown junction. A commonly used structure utilizes a body of P type conductivity material having a relatively low level of doping. A first region of N type conductivity materal extends downwardly from a planar surface of the body. A second region of highly doped P type conductivity material is within the body and beneath the first region thereby forming a PN junction at the area of contact of the first and second regions. This PN junction forms the rectifying or zener breakdown junction of the diode, the junction being substantially parallel to the planar surface of the bulk material. For a more detailed description of a diode having this structure, see U.S. Pat. No. 3,345,221 which issued Oct. 3, 1967 to Lisk.
Another structural arrangement of the P and N regions of a diode utilizes a layer of semiconducting material that is attached to an insulating substrate. Adjacent P and N regions are formed in the semiconducting material so that they penetrate the entire thickness of the layer. This arrangement yields a rectifying junction that is essentially perpendicular to the substrate. See, for example, U.S. Pat. No. 3,513,042 which issued May 19, 1970 to Hagon which illustrates this type of structure.
Zener diodes generally have a breakdown voltage that is accurately determinable, stable, and reproducible. The actual voltage at which breakdown occurs varies over a finite range of values which are related to the physical structure of the device. Usually, at lower current levels, breakdown begins at a single point in the junction. As the current increases the voltage increases causing the breakdown phenomenon to spread until the entire junction is conducting current. At this point the voltage has stabilized at a value slightly higher than that when breakdown first began.
While it is beneficial to increase current at the breakdown junction to effect breakdown of the entire junction, it is also desirable to maintain the total current passing through the device at a low level to keep power consumption within reasonable limits. One way to accomplish this is to reduce the area of the breakdown junction while maintaining the total current passing through the device at a fixed level sufficiently high that the entire junction is conducting. The reduced area requires less current to assure that the entire junction is conducting. The present invention achieves this by means of a novel structure which is insensitive to substantial mask misalignment during manufacture.